"It is not the campaign's policy to attack leaders from other countries," said Howard Wolfson, Mrs Clinton's communications director.

Jamie Rubin, a former assistant secretary of state in Bill Clinton's administration, lashed out at Lord Trimble, who shared the Nobel Peace Prize in 1998, for having said that Mrs Clinton was a "wee bit silly" for exaggerating her role in bringing peace to Northern Ireland.

Related Articles

Mr Rubin said in an interview with MSNBC television's Andrea Mitchell that Lord Trimble was "pretty much the only one" who made the claim about the former First Lady, who says she was "instrumental" in the peace process.

"He's a Protestant, they traditionally go with the Conservatives." He added: "David Trimble is a crankpot and what he said about her was demeaning. He said, 'Oh well, maybe she accompanied her husband on a couple of trips'. As a woman, Andrea, I would think you would recognise when somebody is trying to demean the activities of a woman."

Lord Trimble said Mr Rubin's attack was probably a sign of desperation.

"I'm just sorry that the Clinton campaigners can't just stick to getting their facts right.

"I usually regard it when somebody resorts simply to vulgar abuse that's a sign that they know they've a weak argument." He added: "In the absence of an explanation from the gentleman, I've just no idea what was in his mind."

Mrs Clinton, who trails her rival Barack Obama among the pledged delegates and the popular vote, is fighting a battle on several fronts to persuade Democrats that she can still win her party's nomination.

Her advisers are mounting ferocious attacks on Mr Obama as weak and inexperienced, courting the "super-delegates" - 796 party officials who vote alongside the pledged delegates at the party convention - and trumpeting the former First Lady's foreign policy credentials.

In a letter to Nancy Pelosi this week 20 of Mrs Clinton's top donors berated the Speaker of the House for suggesting that the super-delegates should follow the will of the voters rather than make an independent judgement.

But Mrs Clinton became badly unstuck when she was forced to admit that she "misspoke" about a 1996 visit to Tuzla in Bosnia in which she had falsely claimed she had landed under sniper fire and had to rush across the landing strip.

Television footage showed her emerging smiling from a C-17 aircraft and walking towards an eight-year-old Bosnian girl who presented her and her daughter Chelsea, then 16, with flowers.

Mrs Clinton is also vulnerable on Northern Ireland. Although John Hume, the former leader of the nationalist Social Democratic and Labour Party who shared the Nobel prize with Lord Trimble, has issued a statement supporting her, others in his party have cast doubt on the notion she was a key figure.

Bertie Ahern, the Irish prime minister, and Gerry Adams, the Sinn Fein leader, have praised her role but have not suggested she took part in negotiations.

When asked by this newspaper about Mr Rubin's remarks, Mr Wolfson said: "There is ample, voluminous support for the fact that she worked hard help ensure peace in Northern Ireland, that she visited Northern Ireland many times, that she met with women on different sides of the conflict together."

While he agreed with Mr Rubin's opinion that Lord Trimble's was a minority view, he distanced the campaign from the personal denigration of the former Ulster unionist Party leader.

"I would more or less agree with the first sentiment. I think that the preponderance of folks that have weighed in have been very, very positive...I would not associate myself with the rest of it."